The latter fetches the repository into your $GOPATH directory along with all the Go dependencies, while the former allows you to work in whatever directory you want. If you're working within GOPATH, you probably want to set this variable:

export GO111MODULE=on

Note: go.skia.org is a custom import path and will only work if used like the examples here.

Generated Code

Some code is generated using go generate with external binaries. First, install the version of protoc referenced in the asset creation script and ensure it is on your PATH before other versions of protoc.